Generally, an image sensor is a semiconductor device that converts an optical image into an electric signal. Image sensors may be classified into charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors.
CMOS image sensors use a switching mechanism for detecting an output through MOS transistors. There is at least one MOS transistor for each pixel. The CMOS technology enables peripheral circuits, e.g., a control circuit and a signal processing circuit, to be simultaneously integrated. A CMOS image sensor consists of a photodiode and a plurality of MOS transistors. CMOS image sensors provide an image by basically converting light, i.e., visible ray incident in the front or rear of an image sensor chip, to an electric signal.
Eventually, it will be necessary to implement an image in three dimensions as the technology develops further. However, the structure of current image sensor chips only images the light incident to the front or rear thereof, which puts a limitation on the realization of 3-dimensional image.